Extremes of leaf venation in the Hawaiian silversword alliance. On the left is Dubautia
latifolia with pronounced reticulate venation typical of dicots. On the right is Wilkesia
gymnoxiphium with parallel venation and few cross-connections. This type of venation
is more typical of monocots, especially grasses. To find such extreme differences in leaf
venation among very closely related species is exceptional.

Above: section of the leaf of Dubautia knudsenii, a species from a mesic forest
habitat. Note the comparatively thin lamina with loosely pack cells and large air spaces
in the mesophyll, and the thin cuticle on the upper and lower epidermis. Below: section of
the leaf of Dubautia menziesii, a species from an arid, open habitat. Note the
comparatively thick lamina with more tightly packed cells and smaller air spaces in the
mesophyll, and somewhat thicker cuticle on the upper and lower epidermis.

Section of the leaf of Argyroxiphium grayanum, a species found primarily in and
around the fringes of bogs. Note the compact tissue and large channels that in the intact
leaf are filled with water-binding pectic compounds.

Section of the leaf of Argyroxiphium kauense, a species found primarily in and
around the fringes of bogs. This leaf is very succulent, consisting of compact tissue
interrupted by large channels filled with pectic compounds. The leaves of the Haleakala
silversword (A. sandwicense subsp. macrocephalum), found in arid, cinder
habitats are even more succulent and have a similar construction with large pectin-filled
channels. These pectic materials appear to be important in maintaining a favorable water
economy in arid habitats and also in bog situations where acid conditions and poor root
development contribute to physiological drought. Additional
Reading.